feat: add legacy-api service

실험·앱 계층 설계서 3.2의 legacy-api(VM 계층). /healthz(프로세스 생존),
/v1/price/{item}(db-primary app.price 조회, DB 불가 시 503), /metrics(Prometheus).
Go 1.27.1, pgx v5.11.0, client_golang v1.24.1. 이미지는 Cloud Build로 Artifact Registry에
올린다(MIG 인스턴스가 VM SA로 pull).

legacy-api/Dockerfile

@@ -0,0 +1,15 @@
+# legacy-api (VM layer of the self-healing lab). Built with Cloud Build into Artifact Registry.
+FROM golang:1.27.1-alpine AS build
+WORKDIR /src
+ENV CGO_ENABLED=0 GOFLAGS=-mod=mod
+COPY go.mod ./
+RUN go mod download
+COPY . .
+RUN go build -trimpath -ldflags="-s -w" -o /out/legacy-api .
+
+FROM alpine:3.22
+RUN apk add --no-cache ca-certificates && adduser -D -u 10001 app
+USER 10001
+COPY --from=build /out/legacy-api /usr/local/bin/legacy-api
+EXPOSE 8080
+ENTRYPOINT ["/usr/local/bin/legacy-api"]